1. Field of the Invention
The present invention is generally directed to balancing current among phases and modules in multi-phase power converters and multi-module power supply systems. More specifically, the present invention is directed to a current sensing scheme for sensing phase currents in a multi-phase power supply and adjusting phase duty cycles accordingly through feedback control.
2. Background
Current trends indicate that future microprocessors will exhibit much heavier loads and require power supplies which can provide higher current at lower voltages. Presently, high current requirements are being met with power supplies configured as multi-phase switching mode converters operated in an interleaf mode. Interleaf operation effectively reduces the current ripple seen by the filter capacitor and makes the ripple frequency be multiples of the switching frequency.
Such multi-phase converters in interleaf mode are known in the art. A multi-phase interleaved converter with two phases is illustrated in FIG. 1A. The dashed lines in FIG. 1A indicate additional phases can be included in this multi-phase configuration. The resulting effective reduction in ripple current from this two-phase interleaved converter configuration is indicated by I.sub.0 as shown in FIG. 1B. A single-phase configuration is also represented above the dashed line in FIG. 1A. The circuit represented above the dashed line in FIG. 1A is commonly referred to as a synchronous buck converter. In contrast to the multi-phase interleaved converter configuration, where I.sub.0 includes the ripple canceling effect of i.sub.1 and i.sub.2 as shown in FIG. 1B, the single-phase configuration results in a large output ripple current where I.sub.0 is equal to i.sub.1. Consequently, single-phase configurations require large and bulky output filter capacitors. It is therefore clear that use of the multi-phase converter circuit in interleaf mode is desirable. Advantages of the multi-phase converter circuit in interleaf mode include a smaller filter capacitance needed for static operation, less RMS current drawn from the input capacitor, lower filter inductance needed to speed up the converter dynamic response, and a spreading of the thermal loads among power semiconductor devices.
With the continuing advancement of personal computers into higher-end applications, the merits of the multi-phase power supply configurations in interleaf mode become increasingly attractive for next generation PC power management. Such applications often employ more than one microprocessor, giving rise to the need for paralleling power supply modules to meet the high current load requirements. However, a limitation in presently available interleaved parallel power technology is in sensing current needs in the larger loads and distributing the current load requirements among the various power supply modules and among the various phases within these modules.
In a multi-phase configuration, it is important that all of the phases share a heavy load. In a configuration of multiple power supply modules, it is likewise important that a heavy load be distributed among the modules. In practical circuits, there are various factors contributing to uneven load sharing, including mismatches in the control timing among phases, variation of the power switch parameters, and different printed circuit board (PCB) layouts among phases or modules. Controlling these factors by narrowing their distributions requires a significant addition in margins and costs to circuit designs.
Conventional approaches in sensing and sharing current load requirements include the use of sensing resistors and current transformers. However, sensing resistors reduce converter efficiency by dissipating too much power. The higher the current requirements in a low voltage, high current converter, the more power the sensing resistor dissipates. Current transformers are bulky and expensive and likewise reduce converter efficiency in low voltage, high current converters.
Accordingly, there exists a need for a simple and inexpensive way to sense the phase currents in multi-phase power supplies and equitably distribute the load current requirements among phases in multi-phase power supply configurations and among power modules in multi-module power supply configurations.